[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in proxy_config

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I23c823f9391b84eb3cb97143bf0482dba64f9101
Reviewed-on: https://chromium-review.googlesource.com/c/1352424
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#611748}